The present invention relates to an objective lens used for an optical pickup device and to the optical pickup device, and in particular, to an objective lens for an optical pickup device and to the optical pickup device.
There has been developed an optical pickup device which can record and reproduce information for optical information recording media of different types such as, for example, DVD and CD, and is used for various uses. The optical pickup device of this kind is required to have various kinds of demands including compactness in structure and low cost.
As a light source for an optical pickup device, a semiconductor laser is generally used, and two light sources including a light source for DVD and that for CD have so far been used, because an optimum wavelength for recording and/or reproduction of information for DVD is 635 nm or about 650 nm and an optimum wavelength for recording and/or reproduction of information for CD is about 780 nm.
However, an increase in cost of an optical pickup device is caused by providing two expensive semiconductor lasers. Therefore, it is requested to throw two semiconductor lasers into one. To satisfy this request, it is conceivable that a light source for DVD is used to reproduce information of CD. In a write-once compact disk (CD-R) developed recently, however, there is a problem that recording or reproduction of information cannot be performed because reflectance of light for DVD whose wavelength is about 635 nm is extremely low and reflected light with sufficient amount of light cannot be obtained accordingly.
With the background mentioned above, there are disclosed in TOKKAIHEI NO. 9-306014 by the applicant of this invention an optical pickup device and an objective lens to be used for the optical pickup device which can conduct recording or reproduction of information for DVD and CD-R by using a wavelength which lies between a wavelength of light for DVD and a wavelength of light for CD.
However, when light with the intermediate wavelength disclosed in TOKKAIHEI NO. 9-306014 is used, there still is a problem that it is difficult to conduct recording or reproduction of information for the compact disk (CD-RW) which is more strict than CD-R from a viewpoint of reflectance and can erase information. In addition, there is a fear that a unit cost of a semiconductor laser with the intermediate wavelength may turn out to be high because of less demand for that semiconductor laser.
In TOKKAIHEI NO. 11-339306, on the other hand, there is disclosed an optical pickup device which can conduct reproduction for DVD by using light with wavelength of 780 nm. However, the conventional technology of this kind is one to eliminate an influence of side lobe that is caused when conducting reproduction of DVD using light with wavelength of 780 nm, and it requires an additional circuit for that purpose, resulting in an increase in cost of the optical pickup device.
For these problems, there is an attempt to create an objective lens that shows excellent aberration even when a light flux with high NA and longer wavelength and conducts information recording and reproduction for DVD and CD, by forming plural optical functional regions on the optical surface of the objective lens in the direction perpendicular to the optical axis, and by using an inner optical functional region for both DVD and CD while using an outer optical functional region for DVD only.
For the attempt stated above, it is preferable that the objective lens is made of plastic materials, for carrying out mass production at lower cost. However, since a change in refractive index caused by ambient temperature is great in the case of plastic materials, it is difficult, even when plural optical functional regions are provided, to correct aberration caused by changes in refractive index under the environmental condition of intense changes in temperatures.
In particular, when NA grows to be higher, the tendency for spherical aberration changes caused by temperature changes to increase becomes more remarkable. In this case, amount of change xcex4SA3 of third-order component of spherical aberration caused by temperature change is expressed by the following expression when NA represents an image side numerical aperture of the objective lens, f represents a focal length, m represents an image forming magnification, and xcex represents a wavelength of the laser light source.
(xcex4SA3/xcex4T)xe2x88x9dfxc2x7(1xe2x88x92m)4xc2x7NA4/xcexxe2x80x83xe2x80x83(1)
It is apparent from the expression (1) that aberration correction is more difficult when numerical aperture NA of an objective lens grows greater, because the spherical aberration shows a tendency to deteriorate in proportion to the fourth power of the numerical aperture NA.
The invention has been achieved in view of the problems in the conventional technology, and its object is to provide an optical pickup device and an objective lens to be used in the optical pickup device which can conduct recording and/or reproduction of information for optical information recording media each being different from others by using a single light source with a relatively long wavelength, without using an additional electric circuit, and can control a decline of efficiency caused by ambient changes.
The objective lens to be used in an optical pickup device described in Structure 1 is represented by an objective lens to be used in an optical pickup device having therein a light source and a light converging optical system including an objective lens to conduct recording and/or reproduction of information by making a light flux emitted from the light source to form an image on an image recording surface through transparent base boards of the first and second optical information recording media, wherein when xcex represents a wavelength of the light source, NA1 represents a numerical aperture on the information recording surface side of the objective lens in the case of recording or reproduction of information for the first optical information recording medium, t1 represents a thickness of the transparent base board of the first optical information recording medium and t2 represents a thickness of the transparent base board of the second optical information recording medium,
xcex greater than 700 nmxe2x80x83xe2x80x83(2)
NA1 greater than 0.65xe2x80x83xe2x80x83(3)
t1 less than t2xe2x80x83xe2x80x83(4)
and a plurality of optical functional regions are formed on an optical surface of the objective lens so that a distance of each optical functional region from the optical axis in the direction perpendicular to the optical axis may be different, and an inner optical functional region closer to the optical axis among the aforesaid optical functional regions is arranged to allow a light flux emitted from the light source to pass through for conducting recording or reproduction of information for the first and second optical information recording media, and an outer optical functional region which is farther than the inner optical functional region from the optical axis among the aforesaid optical functional regions is arranged to allow a light flux emitted from the light source to pass through for conducting recording or reproduction of information for the first optical information recording medium, and a diffractive structure is formed on the outer optical functional region.
The invention makes it possible to conduct recording or reproduction of information effectively even when CD-R or CD-RW is used for the second optical information recording medium, for example, because wavelength xcex of the light source exceeds 700 nm. On the other hand, when DVD is used as the first optical information recording medium, for example, there is a fear that an amount of light necessary for recording or reproduction of information may be insufficient for light whose wavelength xcex exceeds 700 nm. In the invention, therefore, the numerical aperture NA1 is made to be greater than 0.65 so that sufficient amount of light may be obtained. Further, for deterioration of spherical aberration caused by a thickness of a transparent base board of the first optical information recording medium and a thickness of a transparent base board of the second optical information recording medium which are different each other, an optimum balance is achieved by the method described below.
FIG. 1 is a partial section showing illustratively an example of the objective lens of the invention related to Structure 1. On optical surface S1 of the objective lens closer to the light source in FIG. 1, an inner optical functional region is formed on the portion within h from the optical axis, and on the portion that is outside the inner optical functional region, there is formed an outer optical functional region. Each of the inner optical functional region and the outer optical functional region is composed of a refractive interface in this case, and a step is formed between them. In the present example, the inner optical functional region is a common area to be used for both the first and second optical information recording media, for example, for both DVD and CD, and by setting the refracting interface of such area based on the medium value of the transparent base boards of both recording media, it is possible to control, in a well-balanced way, the spherical aberration in the light flux that has passed through the area, no matter which medium is used. On the other hand, the outer optical functional region is an exclusive area only for the first optical information medium, for example, for DVD, and by setting the refracting interface of such area based on transparent base board thickness t1 of the first optical information recording medium, it is possible to control properly the spherical aberration in the light flux for conducting recording or reproduction of information for the aforesaid medium. Further, since the diffractive structure is formed on the outer optical functional region, deterioration of aberration caused by changes in the refractive index in accordance with temperature changes in the case of conducting recording or reproduction of information for the first optical information recording medium can be controlled.
In the objective lens of the optical pickup device described in Structure 2, the aforesaid optical functional regions in quantity of three or more are provided, and an intermediate optical functional region is formed between the inner optical functional region and the outer optical functional region.
FIG. 2 is a partial section showing illustratively an example of the objective lens of the invention related to Structure 2. In FIG. 2, on optical surface S1 of the objective lens closer to a light source, an inner optical functional region is formed on a portion from the optical axis up to height h1, an outer optical functional region is formed on a portion that is outside of distance h2 from the optical axis, and an intermediate optical functional region is formed between the aforesaid two areas. Each optical functional region is composed of a refracting interface respectively in this case, and a step is formed between them.
Even in the present example, the inner optical functional region is a common area to be used for both the first and second optical information recording media, for example, for both DVD and CD, and by setting the refracting interface of such area based on the medium value of the transparent base boards of both recording media, it is possible to control, in a well-balanced way, the spherical aberration in the light flux that has passed through the area, no matter which medium is used. On the other hand, the outer optical functional region is an exclusive area only for the first optical information medium, for example, for DVD, and by setting and by setting the refracting interface of such area based on transparent base board thickness t of the first optical information recording medium, it is possible to control properly the spherical aberration in the light flux for conducting recording or reproduction of information for the aforesaid medium. Further, the intermediate optical functional region is an exclusive area only for the second optical information medium, for example, for CD, and by setting the refracting interface of such area based on transparent base board thickness t2 of the first optical information recording medium, it is possible to control properly the spherical aberration in the light flux for conducting recording or reproduction of information for the aforesaid medium. Incidentally, it is also possible to provide a diffractive structure on at least one of the intermediate optical function area and the inner optical functional region. Further, though the intermediate optical function area is used as an exclusive area for CD, it is also possible to divide the intermediate optical function area into plural portions in the direction perpendicular to an optical axis, and thereby to use them as an exclusive area for CD or an exclusive area for DVD.
The objective lens of the optical pickup device described in Structure 3 is characterized to satisfy the following expressions.
NA1 less than 0.95xe2x80x83xe2x80x83(5)
xcex less than 1000 nmxe2x80x83xe2x80x83(6)
The objective lens of the optical pickup device described in Structure 4 is characterized to satisfy the following expressions when NA2 represents a numerical aperture of the objective lens on the information recording surface side in the case of conducting recording or reproduction of information for the second optical information recording medium.
0.3 less than NA2 less than 0.55xe2x80x83xe2x80x83(7)
The objective lens of the optical pickup device described in Structure 5 can be manufactured at low cost on a mass production basis, when it is made of plastic materials.
With respect to the objective lens of the optical pickup device described in Structure 6, when the optical functional regions are formed in a way that a step is given to each boundary portion between the optical functional regions, an operation for a discontinuous amount of spherical aberration is easy, exemplifying that separation between main spot light and flare light on the image recording surface of an optical information recording medium can be made great. Incidentally, in the intermediate optical functional region, it is preferable for the purpose of aberration correction for the second optical information recording medium that a step at the boundary portion that is farther from an optical axis is made to be greater than that at the boundary portion that is closer to an optical axis.
An optical pickup device described in Structure 7 is represented by an optical pickup device having therein a light source and a light converging optical system including an objective lens for conducting recording and/or reproduction of information by making a light flux emitted from the light source to form images on the information recording surface through a transparent base board on each of the first and second optical information recording media, wherein the following expressions are satisfied when xcex represents a wavelength of the light source, NA1 represents the numerical aperture of the objective lens on the information recording surface side in the case of conducting recording or reproduction of information for the first optical information recording medium, t1 represents a thickness of a transparent base board of the first optical information recording medium and t2 represents a thickness of a transparent base board of the second optical information recording medium,
xcex less than 700 nmxe2x80x83xe2x80x83(2)
NA1 less than 0.65xe2x80x83xe2x80x83(3)
t1 less than t2xe2x80x83xe2x80x83(4)
a plurality of optical functional regions are formed on the optical surface of the objective lens to be different in terms of distance from an optical axis in the direction perpendicular to the optical axis, the inner optical functional region closer to the optical axis among the optical functional regions is arranged to allow a light flux emitted from the light source to pass through for conducting recording or reproduction of information for the first and second optical information recording media, the outer optical functional region that is farther from the optical axis than the inner optical functional region among the optical functional regions is arranged to allow a light flux emitted from the light source to pass through for conducting recording or reproduction of information for the first optical information recording medium, and a diffractive structure is formed on the outer optical functional region. An action and an effect of the present invention are the same as those in the invention described in Structure 1.
In the optical pickup device described in Structure 8, the aforesaid optical functional regions in quantity of three or more are provided, and an intermediate optical functional region is formed between the inner optical functional region and the outer optical functional region. An action and an effect of the present invention are the same as those in the invention described in Structure 2.
The optical pickup device described in Structure 9 is characterized to satisfy the following expressions.
NA1 less than 0.95xe2x80x83xe2x80x83(5)
xcex less than 1000 nmxe2x80x83xe2x80x83(6)
The optical pickup device described in Structure 10 is characterized to satisfy the following expression when NA2 represents the numerical aperture of the objective lens on the information recording surface side in the case of conducting recording or reproduction of information for the second optical information recording medium.
0.3 less than NA2 less than 0.55xe2x80x83xe2x80x83(7)
The optical pickup device described in Structure 11 is characterized in that the objective lens is made of plastic materials. An action and an effect of the present invention are the same as those in the invention described in Structure 5.
The optical pickup device described in Structure 12 is characterized in that each optical functional region is formed to have a step at a portion that is a boundary with an adjacent optical functional region. An action and an effect of the present invention are the same as those in the invention described in Structure 6.
In the present specification, when an xe2x80x9coptical functional regionxe2x80x9d is expressed with spherical aberration, let is be assumed that a different optical functional region is present when the expression comes under either one of the following conditions.
(a) When the xe2x80x9coptical functional regionxe2x80x9d is discontinuous at the boundary represented by h (FIG. 3(a))
(b) When the xe2x80x9coptical functional regionxe2x80x9d is continuous, but first order differential is discontinuous (FIG. 3(b))
(c) When the xe2x80x9coptical functional regionxe2x80x9d is continuous at h in a certain wavelength (FIG. 3(c))
An area that is divided under the aforesaid condition and allows each light flux to pass through is regarded as an xe2x80x9coptical functional regionxe2x80x9d. Therefore, when one surface of a lens is looked, if there exist a refracting portion and a diffracting portion (diffractive structure), the refracting portion and the diffracting portion are assumed to be different and separate xe2x80x9coptical functional regionsxe2x80x9d with a boundary portion serving as a border between the refracting portion and the diffracting portion (see FIGS. 1 and 2 ). Further, when diffractive structures designed for different objects are made to be present mixedly under the condition that diffractive structures are formed on the entire surface, they are assumed, from the condition of (c) above, to be different and separate xe2x80x9coptical functional regionsxe2x80x9d. Further, even when an aspheric surface expressed by the same aspherical coefficient is formed on the surface on one side, they are assumed to be different and separate optical surfaces even when a discontinuous portion is formed on the surface on the other side.
Incidentally, xe2x80x9couter optical functional regionxe2x80x9d means an outer optical functional region within an effective diameter, and even when another optical surface is provided within a range that does not affect on recording or reproduction for an optical information recording medium substantially on a partial area in that optical functional region, the invention is not affected, and even if a partial optical surface exists in an effective diameter, such partial optical surface should be ignored and should be considered to be nothing, which applies also to other optical functional regions.
The xe2x80x9cdiffractive structurexe2x80x9d used in the present specification means a portion on the surface of an objective lens which is provided with a relief to have the function to converge or diverge a light flux through diffraction. With respect to a form of the relief, it includes a known form of ring-shaped zones which are mostly in the form of concentric circles whose center is an optical axis and are formed on the surface of an objective lens as shown in FIGS. 1 and 2, for example, each looking like a saw-tooth in a sectional view when viewed on a plane including the optical axis, and this form is especially called xe2x80x9cdiffractive ring-shaped zonexe2x80x9d.
In the present specification, it is assumed that the objective lens in a narrow sense is a lens having a light-converging function arranged to face an optical information recording medium at the position closest to the optical information recording medium under the condition that the optical information recording medium is loaded in the optical pickup device, and the objective lens in a broad sense is a lens capable of being operated, by an actuator, at least in its optical axis direction together with the aforesaid lens. In the present specification, therefore, numerical aperture NA of the objective lens on the optical information recording medium side (image side) means numerical aperture NA of the lens surface of the objective lens positioned to be closest to the optical information recording medium. Further, necessary numerical aperture NA in the present specification is the numerical aperture of the objective lens having diffraction limit performance which makes it possible to obtain a spot diameter necessary for conducting recording or reproduction of information in accordance with a wavelength of a light source to be used, for the numerical aperture specified by the standard of each optical information recording medium or for each optical information recording medium.
In the present specification, the second optical information recording medium means optical disks of various CD types such as, for example, CD-R, CD-RW, CD-Video and CD-ROM, while, the first optical information recording medium means optical disks of various DVD types such as DVD-ROM, DVD-RAM, DVD-R, DVD-Rw and DVD-Video. In addition, thickness t of a transparent base board in the present specification includes t=0.